The present invention relates to mixing devices and more particularly pertains to apparatus for mixing and blending highly viscous fluids or slurries such as drilling fluids, concrete, petroleum crudes, aqueous oleaginous emulsions and the like.
Mixing devices, sometimes known as mud mixers in the drilling industry, are used to prevent separation and viscosity changes of highly viscous fluids such as drilling fluids which create problems for conventional mixing devices in that they are unable to accommodate the varying changes in densities. For example, it is not unusual for drilling fluids to separate into various layers whereupon the heavier, more dense fluids settle to the bottom of the reservoir whereas the lighter fluids rise to the top. In such a situation, it is desirable that the fluid be kept at an overall consistent, uniform density and therefore, it becomes important for such mixing devices to be used in order to insure the uniform density and proper agitation thereof. Due to the large range of densities of fluids encountered, such mixing devices as in the prior art can experience overload conditions which can and are detrimental to the working life of such a mixing device. For example, if a very dense liquid or fluid is encountered, the motor of the mixing device may become overloaded, causing breakdowns, or at a minimum, the motor will be stressed to such a point that it useful life is severely reduced. Of course, if the mixing device fails to operate, the drilling operations must be stopped and, as is well known, maintaining an idle drilling operation is very costly.
Prior art mixing devices include such mixers as disclosed in U.S. Pat. Nos. 2,809,816 and 2,284,765. In some instances, prior art devices such as U.S. Pat. Nos. 4,004,786; 1,995,465; and, 1,450,326 disclose the use of multiple mixing blades and deflection plates for insuring suitable mixing within the mixing tank. In attempts to insure thorough mixing within the tank, deflection plates my be mounted with a rotating shaft of the mixers, as in the above-cited patents, or within and in attachment with the inner sides of the tank as in U.S. Pat. Nos. 3,709,664; 3,222,141; and, German Pat. No. 2,104,380.
Similarly, in attempts to overcome problems of the significant torques encountered in the drilling fluids, some prior art patents are directed towards varying the angle of attack of the mixing blades as in U.S. Pat. Nos. 3,966,176; 3,920,227; and, 3,516,646. So far as known, no attempt has been made to directly control or limit the amount of torque that the motor for such a mixing device must absorb in such a fashion to prevent overloading of the motor upon encountering significantly dense fluids.